Python 2.x return values for cmp

Question

Quoted from the docs:

cmp(x, y)

Compare the two objects x and y and return an integer according to the outcome. The return value is negative if x < y, zero if x == y and strictly positive if x > y.

I was under the assumption that the return values are always -1, 0, and 1 but the docs don't explicitly say that, only mentioning zero and positive/negative return value.

Are there any situations when the return value of cmp(x,y) is not -1, 0, or 1?

Answer 1

No, the docs explicitly say that yalues can be anything. The only value that is specified is 0 if the compared objects are equal. Don't trust the fact that you only see the values -1, 0 and 1, that's an implementation detail and could change*, so always check for < and > 0.

*: note - actually, it won't really have a chance to change, since cmp has gone away in pyhton3. use rich comparison instead.

Answer 2

Only 0 can be relied on, although the docstring for list.sort is interesting:

print list.sort.__doc__
L.sort(cmp=None, key=None, reverse=False) -- stable sort *IN PLACE*;
cmp(x, y) -> -1, 0, 1

But in fact, sort doesn't actually impose this on it's comparison function as can be seen here:

def mycmp(a, b):
    print "mycmp(): a - b = ", a - b
    return a - b

L = [50, 30, 20, 40, 70, 50]

>>> L.sort(cmp=mycmp)
mycmp(): a - b =  -20
mycmp(): a - b =  -10
mycmp(): a - b =  20
mycmp(): a - b =  10
mycmp(): a - b =  -10
mycmp(): a - b =  30
mycmp(): a - b =  20
mycmp(): a - b =  10
mycmp(): a - b =  -20
mycmp(): a - b =  0

>>> print L
[20, 30, 40, 50, 50, 70]

Just for interest, but clearly sort is working happily with values other than -1, 0, or +1, so you should be too!